1. Field of the Invention
The present invention is generally directed to the use of organic compounds as tracers in monitoring the flow of fluids and in particular heavy oil. In one embodiment the flow of heavy oil in a subterranean formation is monitored and in another embodiment the flow of heavy oil through a tank and pipe system is monitored. In yet a third embodiment the flow of oil from different zones in a producing well may be monitored.
2. Background
Tracers are used in the petroleum industry in order to track fluid movement and monitor the changes in flow and composition in subterranean formations. The monitoring of the flow through a formation is particularly important in managing and maximizing production during thermal recovery operations. Tracers may be added to either the water/brine phase or the steam that is injected into the formation through an injection well. Examples include halide salts, boron salts, silicates, and other inorganic and organic water soluble salts. Oil soluble tracers may be injected as part of a "slug" of oil in an injection well. Examples of oil soluble tracers include oil soluble compounds of transition or rare earth metals or highly fluorescent compounds. The principle problem with the use of such compounds is that the technology used to detect the presence of these compounds in samples, (i.e. atomic absorption, atomic emission, ultra-sensitive spectrofluorometry, Internally Coupled Plasma-Mass Spectroscopy (ICP-MS), etc.) is expensive, not suitable for field use and requires highly trained technicians.
Radio-isotopically labeled water soluble or oil soluble compounds have also been used a tracer compounds. These tracers are injected into the subterranean formation using an injection well in a similar manner as the corresponding non-radioactive tracers. The flow of oil in the subterranean formation is determined by monitoring samples taken from a production well and determining the concentration of radioisotope is determined in each sample. From such data, a time verses concentration curve provides information as to the flow characteristics of the subterranean formation. Examples of radio-isotopically labeled tracers include a .sup.131 I labeled iodoethanol, .sup.59 Fe labeled dicyclopentadienyl iron (II) (i.e. .sup.59 Fe-ferrocene) amongst others.
Although the use of radio-isotopically labeled tracers is very sensitive to low concentrations of the tracer in the sample, there are several problems with the use of these tracers. First is the handling and environmental concerns of using and detecting a radio-isotope. Often this requires specialized training and permits from governmental authorities. Cost is another concern since radio-isotopically labeled compounds are expensive and they are not typically recoverable from the produced fluids. Further, the injection of radio-isotopically labeled tracers "contaminates" the entire formation with a radio-isotope which alters the natural isotopic composition of the produced fluids. Such contamination may interfere with the scientific analysis of the produced fluid's characteristics. In particular, the use of radio-isotopically labeled tracers may interfere in the radio isotope dating of the formation. Finally, and not the least of importance, the use of a radio-isotope labeled tracer may be limited by the unduly short or excessively long half life of the radio-isotope.
Much of the work in tracer compounds for thermal recovery operations has focused on water or brine soluble tracers since such compounds can be easily added to the water, brine or steam injected into the formation during a water injection, steam flood, or other thermal recovery operations. Examples of such tracers may be found in U.S. Pat. No. 5,246,860 which discloses the use of carboxylic, and sulfonic acids and their salts as suitable tracers. Although oil soluble tracers are disclosed, they are limited to aniline and pyridine compounds that upon addition of acid form water soluble salts.
Despite the above developments in tracer technology, there exists a continuing need for oil soluble tracers, especially those that can be used in monitoring the flow of heavy oil during a steam flood or other thermal recovery process.